Anomalous diffusion of a penetrant in a viscoelastic polymer: Modelling and testing

It is now well known that Fick's Law is frequently inadequate for describin g penetrant diffusion in polymers or polymer composites. Non-Fickian or ano malous diffusion typically occurs when the rates of diffusion and viscoelas tic relaxation in a polymer are comparable, and the ambient temperature is below the glass transition temperature (T-g) of the polymer. As a result, i t is necessary to take into account the time-dependent response of a polyme r, analogous to viscoelastic relaxation of mechanical properties, in constr ucting such a model. In this paper, a simple yet robust methodology is proposed that would allow characterization of non-Fickian diffusion coefficients from penetrant weig ht gain data for a polymer below its T-g: Reduced absorption plots are used to verify the thickness independence of the diffusivity data. Subsequently , these diffusion coefficients are used for predicting penetrant concentrat ion profiles through the thickness of a polymer. Penetrant weight gain data at different temperatures for an epoxy and a urethane adhesive specimen is employed to test the model.